Technical Field
This disclosure generally relates to distortion compensation, and more particularly to circuits and methods for correcting distortion for in phase and quadrature component signals.
Description of Related Art
Communication systems include various circuits, such as receiver demodulators and baseband amplifiers, which have nonlinearities that may lead to “unwanted signals.” The nonlinear behavior of such circuits induces intermodulation products that, together with the system noise, increase the amount of signal distortion, thereby lowering the system performance. Unwanted signals that occur at multiples of the input frequency are referred to as harmonic distortion (HD). Unwanted signals that occur at frequencies that are linear combinations of the input frequencies are referred to as intermodulation distortion (IM). Nonlinear distortion products include a 2nd Order Intermodulation Term (IM2), a 2nd Order Harmonic Distortion Term (HD2), a 3rd Order Intermodulation Term (IM3), and a 3rd Order Harmonic Distortion Term (HD3).
There have been attempts to remove or compensate for the distortions by using the in-phase I or quadrature Q baseband signals to directly calculate distortion products that are then removed from the same in-phase or quadrature baseband signals. For example, U.S. Pat. No. 8,848,824 uses a complex system that may only correct for odd-order nonlinearities, such as IM3. It uses non-linear distortion generators driven appreciably into their non-linear regions. The compensating distortion signals are generated from the in-phase or quadrature path independently.
In U.S. Pat. No. 8,050,649, an RF IM2 generator is configured to only correct for mixer IM2 distortion. The '649 patent is not readily applicable to baseband signal paths and cannot be readily used to correct for both mixer baseband distortion and baseband amplifier distortion. The method of the '649 patent uses the RF signal and a non-linear distortion generator to produce an IM2 signal to sum into the baseband. Because of such summation, distortion generated in the baseband path may not be cancelled.
In U.S. Pat. No. 8,483,646 the common-mode to differential-mode feedback is configured to correct for only IM2. The method of the '646 patent relies on the common-mode IM2 distortion products already present in the mixer output to generate a differential IM2 compensation signal. If the mixer already has low common-mode IM2 distortion products, there may not be enough IM2 input signal to generate the required output differential IM2 compensation signal.
U.S. Pat. No. 7,876,867 represents an approach where DSP methods are used to calculate distortion components (i.e., an analog solution is not used). The method of the '867 patent uses a digital IM2 generator that calculates the squared magnitude of the I and Q signals, and generates an IM3 signal based on this digital IM2. Since the '867 patent uses a digital approach, it does not have the potential precision that is offered by calculating IM2 distortion in the analog domain. The methods of the '824, '649, and '646 patents may not allow for independent control of all of the distortion components, namely IM2, HD2, IM3, HD3. There are many older approaches that involve feed-forward or pre-distortion and make use of delay lines and couplers, which cannot be easily integrated on chip. It is with respect to these considerations and others that the present disclosure has been written.